In a medium saucepan stir together water, lentils, onion, cilantro, salt, cumin, and garlic. Bring to boiling. Reduce heat and simmer, covered, for 12-15 minutes or until lentils are tender and most of the liquid is absorbed. Use a fork to mash the cooked lentils.
Spread the lentil mixture on tostada shells, top with vegetables and cheese.
Place on a large baking sheet.
Broil 3-4 inches from the head for about 2 minutes or until the cheese melts.